Rosenbaum vs. Snellen

[baya]

I'm a medical student about to start my intro to ophtho rotation on Monday. I feel that my medical education in ophtho prior to this point has been pretty minimal, so I've done a little reading to prepare. But I don't understand something, and was hoping folks on this forum might be able to elucidate the matter for me.

Now of course a Snellen chart is read at 20 feet while a Rosenbaum card is read at 14 inches, but I had previously thought that they were "equivalent", and one would get the same VA reading, so long as they were each read at the appropriate distance.

However, one of my books says that a Snellen chart is for determining "visual acuity at far", and a Rosenbaum chart is for determining "visual acuity at near".

It goes on to say that if only one of them is poor, the problem is definitely refractive, while if both are poor the problem may be refractive or non-refractive.

Then it gives an example in which a myopic person reads 20/20 on Rosenbaum but 20/100 on the Snellen, and a presbyopic person reads 20/100 on Rosenbaum but 20/20 on the Snellen. A person with cataracts reads 20/100 on both the Rosenbaum and on the Snellen.

I'm a little bit suspicious about this, mainly because I'd always been led to think that the two cards are equivalent (I understand that they are equivalent in that they project the same sized image onto the retina). Is the book right? Does anyone have any insight into the best way to understand the optics of this?

TIA

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[Caffeinated]

I'm a medical student about to start my intro to ophtho rotation on Monday. I feel that my medical education in ophtho prior to this point has been pretty minimal, so I've done a little reading to prepare. But I don't understand something, and was hoping folks on this forum might be able to elucidate the matter for me.

Now of course a Snellen chart is read at 20 feet while a Rosenbaum card is read at 14 inches, but I had previously thought that they were "equivalent", and one would get the same VA reading, so long as they were each read at the appropriate distance.

However, one of my books says that a Snellen chart is for determining "visual acuity at far", and a Rosenbaum chart is for determining "visual acuity at near".

It goes on to say that if only one of them is poor, the problem is definitely refractive, while if both are poor the problem may be refractive or non-refractive.

Then it gives an example in which a myopic person reads 20/20 on Rosenbaum but 20/100 on the Snellen, and a presbyopic person reads 20/100 on Rosenbaum but 20/20 on the Snellen. A person with cataracts reads 20/100 on both the Rosenbaum and on the Snellen.

I'm a little bit suspicious about this, mainly because I'd always been led to think that the two cards are equivalent (I understand that they are equivalent in that they project the same sized image onto the retina). Is the book right? Does anyone have any insight into the best way to understand the optics of this?

TIA

It sounds like you have it figured out. I'm not quite sure where the question is. As for the optics, the nearsighted person can focus at near without accommodating. The presbyopic person (who is otherwise emmetropic) cannot accommodate to see clearly at near, but can see at distance. For these first two examples, the problem is with the focus of the visual system. For the cataract example, it's not a focus issue, it's a problem with the quality of the optical media (in this case, the crystalline lens). Other examples of optical media problems include corneal opacity, vitreous hemorrhage, to name a few.

Again, sounds like you have it figured out, but you just need to convince yourself that you understand it.

 

[baya]

It sounds like you have it figured out. I'm not quite sure where the question is. As for the optics, the nearsighted person can focus at near without accommodating. The presbyopic person (who is otherwise emmetropic) cannot accommodate to see clearly at near, but can see at distance. For these first two examples, the problem is with the focus of the visual system. For the cataract example, it's not a focus issue, it's a problem with the quality of the optical media (in this case, the crystalline lens). Other examples of optical media problems include corneal opacity, vitreous hemorrhage, to name a few.

Again, sounds like you have it figured out, but you just need to convince yourself that you understand it.

Okay, I guess I just wanted confirmation because the book that told me this was "Ophthalmology Made Ridiculously Simple", and I hadn't heard it anywhere else, so I wasn't sure whether to believe it. I feel like I should have been told this basic information sooner in my medical education. So if you're saying the above is correct, I'll go ahead and believe it.

Thanks for the confirmation.